1. Field of the Invention
The present invention generally relates to a two-cycle combustion engine of an air scavenging type suitable for use as a power plant for a compact work machine such as, for example, a bush cutter or mowing machine.
2. Description of the Prior Art
The two-cycle combustion engine of the type referred to above has hitherto been well known, in which prior to the air-fuel mixture introduced into the combustion chamber to scavenge the latter, preparatory scavenging with air takes place to suppress a blow-off of a portion of the air-fuel mixture outwardly through the exhaust port together with combustion gases. Specifically, the conventional two-cycle combustion engine of this type includes first and second scavenge passages each defined in part in the engine cylinder and in part in the crankcase so that air can be once introduced into the second scavenge passage and then supplied into the combustion chamber through the second scavenge passage prior to the air-fuel mixture being supplied into the combustion chamber through the first scavenge passage during the power and exhaust stroke.
In this conventional two-cycle combustion engine, the second scavenge passage is fluid connected by means of a connecting tube and clamps with an air supply passage through which the air is introduced from the outside of the combustion engine, for example, from the atmosphere by way of an air cleaner unit. Because of this, the number of component parts such as the connecting tube and clamps as well as the number of assembling steps increases, resulting in reduction in productivity and increase in manufacturing cost.
On the other hand, the Japanese Laid-open Patent Publication No. 2001-193557 discloses another conventional two-cycle combustion engine of the structure, in which an air supply chamber is defined in the wall of the engine cylinder adjacent an air intake system and is fluid connected with a pair of air branch passages defined also in the wall of the engine cylinder so that the preparatory scavenging can be accomplished with air supplied from the air supply chamber. This conventional two-cycle combustion engine is advantageous in that the number of component parts such as the connecting tube and the clamp can be reduced considerably, and the number of assembling steps can also be reduced, accompanied by increase in productivity.
However, in the case of the conventional two-cycle combustion engine disclosed in the above mentioned patent publication, the air branch passages in the wall of the engine cylinder are each formed by means of a molding technique utilizing dies or molds of a complicated shape. In other words, referring to FIG. 4 of the above mentioned patent publication it is clear that the air branch passages in the wall of the engine cylinder are each formed so as to extend from the air supply chamber with their respective longitudinal axis inclined relative to the longitudinal axis of the engine cylinder and, therefore, the use of the dies or molds of a complicated shape is essential during the dies molding process. Considering that the dies or molds of a complicated shape are generally expensive, the two-cycle combustion engine disclosed in the above mentioned patent publication obviously requires increase of the manufacturing cost.